130 H. H. NEWMAN. 



of a mere parasite upon the normal component. Thus arises the 

 condition known as that of the "autosite and parasite" type of 

 double monsters. In extreme cases of this sort the "parasite" 

 component is reduced to a rudimentary cyst embedded in the 

 tissues of the parasite. This condition I have already interpreted 

 as one of dominance and subordination in growth. According to 

 this principle, which has been shown to hold good in large 

 numbers of cases in both animals and plants, a rapidly growing or 

 differentiating region, or apical region, in an organism tends to 

 inhibit the growth or differentiation of like regions within the 

 realm of its influence. This is true of equivalent bilateral 

 regions, which may be conceived of as intense rivals. If one rival 

 region gains an ascendancy over the other, the inferior region 

 tends to be suppressed. 



Applying this principle to echinoderm development, we can 

 readily recognize the analogy to the "autosite and parasite" 

 situation in double monsters. Commonly the left side acts as the 

 superior or "autosite" component, and the right side as the 

 inferior or "parasite" component. The result is that certain 

 important and actively differentiating structures (the posterior 

 coelom, the hydropore, and the hydroccele) appear first on the 

 more rapidly developing left side, and that these structures sup- 

 press the development of equivalent structures on the right side 

 and tend to reduce to vestiges certain other coelomic structures 

 of the right side that have already appeared. In other words, the 

 typical adult echinoderm is morphologically the surviving left- 

 hand component of a twin or double monster, and its radial 

 symmetry is merely a sort of mechanical adjustment necessary in 

 order to avoid too pronounced a one-sidedness. 



If, however, the rate of growth of a larva be sharply checked in 

 such a way that the physiological state of the right and left 

 components are reduced to a parity, it should be possible to get 

 an organism both halves of which are equivalent. As a matter of 

 fact, this result is not infrequently realized in the bilateral larvae 

 that have arisen as the result of rather prolonged icing. The 

 commoner result, however, is that the right side usurps the 

 ascendancy typically belonging to the left side and gives rise to 

 larve with reversed asymmetry. If the physiological condition 



